Illumination optical system and projection display optical system

ABSTRACT

An illumination optical system is disclosed, which provides a luminous flux with a small incident angle on an illumination surface in one axis direction on a section of the luminous flux. The illumination optical system can suppress a reduction in light amount by a mask provided for a polarization conversion element. The illumination optical system has a light source and an optical integrator. The optical integrator uses a lens array to perform splitting of a luminous flux from the light source. The illumination optical system has the polarization conversion element including a polarization beam splitter array, a plurality of ½ wave plates, and a mask. The light source is a discharge gas exciting arc tube of a DC drive type.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an illumination optical systemand a projection display optical system which are used in a projectiondisplay apparatus or the like.

[0003] 2. Description of Related Art

[0004] Conventionally, in a projector type display (a projection displayapparatus), a liquid crystal display panel or a micromirror array devicepanel is typically used as a light modulation element for switching tocontrol transmission and shielding or deflection of light to project aselected light pattern onto a screen, thereby displaying an image on thescreen.

[0005] In the projector which employs the liquid crystal display panelor the micromirror array device panel as the light modulation element,it is important to use light from a light source with high efficiencyand reduce variations in illuminance on the screen.

[0006] An optical integrator formed of two lens arrays each includinglenses arranged two-dimensionally is a known means for improvement. Inthe optical integrator, a first lens array splits a luminous flux from alight source into a plurality of luminous fluxes, and a second lensarray and a condenser lens enlarge the luminous fluxes and form imagesby the luminous fluxes superimposed one on another on a display area ofa light modulation element (see Japanese Patent Application Laid-OpenNo. 11(1999)-64848).

[0007] In this method, since the split luminous fluxes with smallvariations in illuminance are superimposed, the resulting irradiationlight has high uniformity to significantly reduce variations inilluminance on the screen. When the first lens array has each apertureformed in a rectangular shape similar to the display area of the lightmodulation element, all the split luminous fluxes are irradiated to thedisplay area without waste. This improves the efficiency of theirradiation light and thus improves the use efficiency of the light fromthe light source.

[0008] Another means for improvement is to guide light from a lightsource to a kaleidoscope to mix the vectors of light rays to provideuniform light intensity distribution at an end surface of thekaleidoscope from which the light emerges, and then form a conjugateimage by an image-forming lens on a micromirror array device used as alight modulation element.

[0009] When the kaleidoscope is used, an optical system is complicatedif a means for converting natural emission light from the light sourceinto linearly polarized light is used. Thus, such a means is not usedgenerally.

[0010] In the method, the resulting irradiation light has highuniformity to significantly reduce variations in illuminance on ascreen.

[0011] However, in the method of providing uniform light intensitydistribution using the optical integrator formed of two lens arrays orthe kaleidoscope, the luminous flux illuminating the light modulationelement has a large convergent angle. When the light modulation panel isrealized by a reflection type liquid crystal display panel or amicromirror array device, limitations are imposed on space for formingan optical path along which the illumination light is guided. When a TIR(total internal reflection tilt) prism is used to guide light, theminimum angle of total reflection is limited. When a polarization beamsplitter is used to guide light, limitations are imposed due todependency of the reflectivity of S waves and transmittance of P waveson the incident angle. From these facts, the illumination luminous fluxincident on the light modulation element is desirably close to acollimated luminous flux.

[0012] In addition, when a transmission type liquid crystal displaypanel is used as the light modulation element to modulate light of treeprimary colors of red, green, and blue, the modulated light componentsare then combined by a dichroic mirror or dichroic prism. In this case,as the modulated light is less similar to a collimated luminous flux,the cut wavelength in a reflection/transmission wavelength region of adichroic film is changed to produce turbidity of colors or variations incolor reproducibility depending on the position of a projected image.

[0013] When twisted nematic liquid crystal (TNLC) is used as the lightmodulation element, whether it is of a transmission type or a reflectiontype, as the incident angle of an illumination luminous flux on theliquid crystal display panel is more inclined with respect to the normalto the panel, and generally, more inclined with respect to each sidedirection in the liquid crystal display panel plane, a larger deviationoccurs from 0 or π which is an ideal phase difference of a wave providedby transmission through the liquid crystal display panel. Therefore,contrast in light modulation is reduced.

[0014] To address this, the present inventors have proposed anillumination optical system which illuminates an illumination surfacewith a generally telecentric illumination luminous flux (which meansthat it includes somewhat divergent and convergent components) in which,in intensity distribution of illumination light on the illuminationsurface changing depending on a deviation angle of an incident ray withrespect to a normal to the illumination surface, a ratio of angle widthsat which light intensity reaches half of a peak value in each of twoaxis directions orthogonal to each other on the illumination surface isan aspect ratio of 2:1 or higher. The illumination optical system has anoptical integrator which performs splitting and recombination on theluminous flux in a first axis direction on a section generallyorthogonal to the traveling direction of the illumination luminous flux,and a light intensity conversion element which performs conversion oflight intensity distribution in a second axis direction orthogonal tothe first axis direction on the section.

[0015] This can realize an illumination optical system which can uselight from a light source with high efficiency and can provide anillumination luminous flux with highly uniform illuminance. Theillumination optical system can be used as an illumination section in aprojection display optical system to provide a projected image with ahigh contrast.

[0016] In the illumination optical system described above, however, whenan incident luminous flux from a light source lamp has large divergence,the use efficiency of light from the light source may be reduced.

[0017] In commercially available full-color projection type displayapparatuses, a color splitting/recombination optical system typicallyhas a color splitting direction set to a horizontal direction of aprojected image. This is because an apparatus in an oblong shape isconveniently handled as a video-related device. For image signals, adisplayed image has a length-to-width ratio of 4:3 as an image displayratio in the NTSC system, or a length-to-width ratio of 16:9 as an imageratio in the MUSE system. A long side direction of a light modulationpanel naturally matches the direction of color splitting/recombinationfor full-color display.

[0018] In other words, the direction of luminous flux splitting in awavelength band splitting film (a dichroic mirror or the like) or apolarization beam splitter corresponds to the long side direction of thelight modulation panel.

[0019] To prevent degraded accuracy of light splitting due to variationsin an incident angle of light on the wavelength band splitting film orthe polarization beam splitter, it is necessary to set a small incidentangle of an illumination luminous flux on the light demodulation panelin the direction of color splitting/recombination, that is, the longside direction of the light modulation panel.

[0020] Thus, in the illumination optical system proposed by the presentinventors described above, the direction of optical integration is setto a short side direction of a light modulation panel.

[0021] In this case, a disadvantage occurs in the use efficiency oflight from the light source lamp. Specifically, the direction of opticalintegration is set to the short side direction of the light modulationpanel, so that the direction of multi-stage arrangement of apolarization conversion element called a PS conversion element which ismainly formed of polarization beam splitters arranged in multiple stagesand half-wave plates is set to the short side direction of the lightmodulation panel. For this reason, the arrangement directions of thelight source and a multi-stage slit mask disposed in the polarizationconversion element, generally disposed in the long side direction of thelight modulation panel, are also set to the short side direction of thelight modulation panel. Consequently, a larger amount of light withdivergence from the light source is shielded by the multi-stage slitmask to reduce light transfer efficiency of the illumination opticalsystem.

SUMMARY OF THE INVENTION

[0022] It is an object of the present invention to provide anillumination optical system which provides an illumination luminous fluxwith a small incident angle on an illumination surface in one axisdirection on a section of the illumination luminous flux and whichsuppresses a reduction in light amount blocked by a mask (that is, areduction in light amount) provided for a polarization conversionelement, a projection display optical system which employs theillumination optical system, a projection display apparatus, and animage display system.

[0023] To achieve the aforementioned object, according to one aspect,the present invention provides an illumination optical system which hasa light source, an optical integrator, and a polarization conversionelement including a polarization beam splitter array, a plurality of ½wave plates, and a mask. The optical integrator uses a lens array toperform splitting (or splitting and recombination) of a luminous fluxincident as a generally collimated luminous flux from the light sourcein a first axis direction in a two-dimensional section orthogonal to atraveling direction of the illumination luminous flux. The polarizationbeam splitter array has a plurality of polarization beam splittersarranged in multiple stages corresponding to a plurality ofpredetermined lens areas in the lens array. Each of the ½ wave platesrotates a polarization direction of first polarized light substantially90 degrees out of the first and second polarized light with polarizationdirections orthogonal to each other split by each of the polarizationbeam splitters. The mask covers a plurality of areas of incidentsurfaces of the polarization beam splitter array to prevent incident ofthe second polarized light on each of the ½ wave plates.

[0024] The present invention according to another aspect provides aprojection display apparatus which comprises a light source which is adischarge gas exciting arc tube of a DC drive type, an opticalintegrator which uses a lens array to perform splitting of a luminousflux incident as a generally collimated luminous flux from the lightsource in a first axis direction in a two-dimensional section orthogonalto a traveling direction of the luminous flux, and a polarizationconversion element. The polarization conversion element includes apolarization beam splitter array, a plurality of ½ wave plates, and amask. The polarization beam splitter array has a plurality ofpolarization beam splitters arranged in multiple stages corresponding toa plurality of predetermined lens areas in the lens array. Each of the ½wave plates rotates a polarization direction of first polarized lightsubstantially 90 degrees out of the first and second polarized lightwith polarization directions orthogonal to each other split by each ofthe polarization beam splitters. The mask covers a plurality of areasout of an incident surface of the polarization beam splitter array toprevent incidence of the second polarized light on each of the ½ waveplates. And the projection optical system further comprises a spatiallight modulator which modulates a luminous flux emerging from theillumination optical system by a group of pixels arrangedtwo-dimensionally, and a projection lens which projects the luminousflux modulated by the spatial light modulator onto a projection surface.

[0025] The present invention according to yet another aspect provides anillumination optical system which comprises a light source in which acathode electrode and an anode electrode are provided, and by applying aDC voltage a discharge gas is excited and light is emitted from thevicinity of the cathode electrode, and a lens array in which a pluralityof lenses are arranged in a first direction substantially orthogonal toan illumination direction. Each of the lenses condenses a part of aluminous flux from the light source in the first direction. Theillumination optical system further comprises a mask in whichlight-transmitting portions transmits luminous fluxes condensed by thelenses and light-blocking portions blocking the luminous fluxescondensed by the lenses are arranged alternately in the first direction.

[0026] The present invention according to yet another aspect provides aprojection display apparatus which comprises a light source in which acathode electrode and an anode electrode are provided, and by applying aDC voltage a discharge gas is excited and light is emitted from thevicinity of the cathode electrode. The apparatus further comprises alens array in which a plurality of lenses are arranged in a firstdirection substantially orthogonal to an illumination direction. Eachlens condenses a part of a luminous flux from the light source in thefirst direction. The apparatus further comprises a mask in whichlight-transmitting portions transmits luminous fluxes condensed by thelenses and light-blocking portions blocking the luminous fluxescondensed by the lenses are arranged alternately in the first direction.The apparatus further comprises a polarization beam splitter array inwhich first polarization beam splitters and second polarization beamsplitters are arranged alternately in the first direction. Each firstpolarization beam splitter reflects a first polarized light out oftransmitted light through the light-transmitting portion and transmits asecond polarized light out of the transmitted light. The polarizationdirection of the second polarized light is rotated by substantially 90degrees from the polarization direction of the first polarized light.Each second polarization beam splitter reflects the first polarizedlight reflected by the first polarization beam splitter in a directionsubstantially parallel to the transmitting direction of the secondpolarized light. The apparatus further comprises wave plates whichrotate the polarization direction of the first polarized light from thesecond polarization beam splitters substantially 90 degrees, and a lightmodulator which modulates the second polarized light at a substantialrectangular area having a short side in the first direction. Theprojection display apparatus further comprises a projection opticalsystem which projects modulated light by the light modulator.

[0027] These and other characteristics of the illumination opticalsystem, the projection display optical system employing the illuminationoptical system, the projection display apparatus, and the image displaysystem of the present invention will be apparent from the followingdescription of specific embodiments with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 schematically shows the structure of an illuminationoptical system which is Embodiment 1 of the present invention;

[0029]FIG. 2 schematically shows the structure of a polarizationconversion element included in the illumination optical system in FIG.1;

[0030] FIGS. 3(A) and 3(B) are schematic diagrams showing a luminousflux from a discharge gas exciting arc tube of a DC drive type in theillumination optical system in FIG. 1 toward the polarization conversionelement;

[0031] FIGS. 4(A) and 4(B) are schematic diagrams showing a luminousflux from a discharge gas exciting arc tube of an AC drive type in theillumination optical system toward the polarization conversion element;

[0032]FIG. 5 is a schematic diagram for explaining the function of anoptical integrator incorporated in the illumination optical system;

[0033]FIG. 6 is a schematic diagram for explaining the function of lightintensity conversion optics incorporated in the illumination opticalsystem;

[0034] FIGS. 7(A) to 7(C) are diagrams for explaining the process ofproducing generally uniform light intensity distribution by theillumination optical system;

[0035] FIGS. 8(A) and 8(B) are graphs for explaining the process ofproducing generally uniform light intensity distribution by theillumination optical system;

[0036] FIGS. 9(A) to 9(C) show light irradiation angle distribution on alight modulation panel by the illumination optical system;

[0037] FIGS. 10(A) to 10(C) show light irradiation angle distribution ona light modulation panel by an illumination optical system using aconventional two-dimensional optical integrator; and

[0038]FIG. 12 is a schematic diagram showing the structure of aprojection display apparatus which is Embodiment 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] In the following, embodiments of the present invention aredescribed with reference to the drawings.

[0040] (Embodiment 1)

[0041]FIG. 1 shows the structure of an illumination optical system whichis Embodiment 1 of the present invention. In FIG. 1, reference numeral200 shows a gas exciting lamp of a DC drive type (a discharge gasexciting arc tube of a DC drive type) serving as a light source. As thelamp 200, a high-pressure mercury lamp, a metal halide lamp, a xenonlamp or the like is used. The light source lamp 200 is used incombination with a parabolic reflecting mirror 201 to produce agenerally collimated visible light beam.

[0042] To provide a high-quality collimated luminous flux with thesmallest possible divergence (the smallest possible divergence angle),the light source lamp 200 has a minimized discharge gap designed tolimit an electron excited area in a gas. A DC bias is applied between acathode and anode electrodes to produce a point source with highluminance on the side (in the vicinity) of the cathode electrode.

[0043] Of the luminous flux emitted from a lamp unit formed of the lightsource lamp 200 and the parabolic reflecting mirror 201, ultravioletrays outside visible light are cut by an ultraviolet ray cut filter 202.

[0044] Optical glass and an optical thin film used as materials of alens are excited by the ultraviolet rays and deterioration of themoccurs in the long term. However, the ultraviolet ray cut filter 202 isprovided mainly for preventing the ultraviolet rays from decomposing andaltering a liquid crystal polymer which is an organic material or apolymer liquid crystal orientation film for arranging liquid crystalmolecules when a liquid crystal element is used for a light modulationpanel (a spatial light modulator).

[0045] The luminous flux of visible light transmitted through theultraviolet ray cut filter 202 is then incident on a first cylindricallens array homogenizer 206. The first cylindrical lens array homogenizer206 has a refractive power only in a vertical direction (first axisdirection) in FIG. 1. The first cylindrical lens array homogenizer 206splits the incident luminous flux into luminous fluxes, the number ofwhich is equal to the number of the lenses of the array, and focal linesare individually formed, and then a cylindrical condenser lens 209converts the luminous fluxes into collimated luminous fluxes set to havea predetermined width.

[0046] The spacing between the principal planes of the first cylindricallens array homogenizer 206 and the cylindrical condenser lens 209 is setto the sum of the focal length of the first cylindrical lens arrayhomogenizer 206 and the focal length of the cylindrical condenser lens209. This causes the luminous fluxes to be converted into collimatedluminous fluxes as described above.

[0047] Since the first cylindrical lens array homogenizer 206 has anoptical axis line decentered with respect to an optical axis line ofeach lens of the array, the cylindrical condenser lens 209 superimposesthe luminous fluxes transmitted through the respective lenses of thefirst cylindrical lens array homogenizer 206 at the position of a focalline of the cylindrical condenser lens 209. This achieves an opticalintegration operation. The position of the focal line of the cylindricalcondenser lens 209 corresponds to a modulation surface of a lightmodulation panel 212.

[0048] The luminous fluxes transmitted through the first cylindricallens array homogenizer 206 are transmitted through a second cylindricallens array homogenizer 207. The positions of focal lines of the secondcylindrical lens array homogenizer 207 are set to the positions ofpupils of the respective lenses of the first cylindrical lens arrayhomogenizer 206. The tandem lens structure of the second cylindricallens array homogenizer 207 and the cylindrical condenser lens 209results in an optically conjugate relationship between the pupils of therespective lenses of the first cylindrical lens array homogenizer 206and the modulation surface of the light modulation panel 212.Consequently, the pupils of the respective lenses of the firstcylindrical lens array homogenizer 206 are imaged on the modulationsurface of the light modulation panel 212 in the vertical direction inFIG. 1.

[0049] The luminous flux emitted from the lamp unit formed of the lightsource lamp 200 and the parabolic reflecting mirror 201 is notcompletely collimated and has divergence. The second cylindrical lensarray homogenizer 207 corrects the divergence of the luminous flux toreliably guide the luminous flux transmitted through the pupil of eachlens of the first cylindrical lens array homogenizer 206 to themodulation surface of the light modulation panel 212.

[0050] The luminous fluxes transmitted through the second cylindricallens array homogenizer 207 are incident on a polarization conversionelement 208. The polarization conversion element 208 is similar to thatcalled a PS conversion element which is generally used in a liquidcrystal projector. The polarization conversion element 208 changes thelight emitted from the lamp unit into polarized light components inparallel with one direction, for example with the vertical direction inFIG. 1, by an array of polarization beam splitters.

[0051] (About Polarization Conversion Element 208)

[0052]FIG. 2 schematically shows the polarization conversion element208. The polarization conversion element 208 is formed of a polarizationbeam splitter array 208 a, a plurality of half-wave plates (½ waveplates) 208 c, and a multi-stage slit mask (hereinafter referred tosimply as a “mask”) 208 d. The polarization beam splitter array 208 aincludes a number of (a plurality of) stages of polarization beamsplitters disposed in the vertical direction, in which each polarizationbeam splitter has a polarization splitting film 208 b inclined 45degrees with respect to an incident optical axis. The ½ wave plate 208 cis provided on an emergent surface of every other polarization beamsplitter (second polarization beam splitter) in the vertical directionof the plurality of polarization beam splitters. The mask(light-blocking portions) 208 d covers each incident surface of thepolarization beam splitter provided with the ½ wave plate 208 c and hasslit-like apertures (light-transmitting portions) 208 e which matchincident surfaces of the polarization beam splitters (first polarizationbeam splitters) provided with no half-wave plate 208 c.

[0053] A P-polarized light component with a polarization direction inparallel with the sheet of FIG. 2 is transmitted through thepolarization splitting film 208 b. An S-polarized light component with apolarization direction orthogonal to the sheet of FIG. 2 is reflected bythe polarization splitting film 208 b and again reflected by thepolarization splitting film 208 b of the polarization beam splitterimmediately below. This causes the optical path of the S-polarized lightcomponent to be shifted downward by a pitch of the polarization beamsplitters arranged in the polarization beam splitter array 208 a.

[0054] The S-polarized light component emerging from the polarizationbeam splitter is given a phase difference of π by the ½ wave plate 208 cand thus the polarization direction thereof is changed substantially 90degrees. The resulting light emerges from the polarization conversionelement 208 as P-polarized light. In this manner, all luminous fluxestransmitted through the polarization conversion element 208 becomelinearly polarized light which is P-polarized light with respect to thepolarization beam splitters. In other words, all the luminous fluxesemerge as polarized waves in parallel with the sheet.

[0055] If the P-polarized light transmitted through the polarizationsplitting film 208 b is incident on the ½ wave plate 208 c, thepolarization direction thereof is rotated substantially 90 degrees toresult in S-polarized light which emerges from the polarizationconversion element 208. To avoid this, the mask 208 d for preventingincidence of luminous fluxes is provided on the incident surfaces of thepolarization beam splitters opposite to the ½ wave plates 208 c to allowincidence of luminous fluxes only in the slit apertures 208 e formed inthe mask 208 d.

[0056] In FIG. 1, the luminous fluxes transmitted through thepolarization conversion element 208 are incident on a first cylindricallens 205. The first cylindrical lens 205 has a refractive power only ina horizontal direction in FIG. 1, and forms a beam compressor as a pairwith a second cylindrical lens 210 disposed in a direction in which theluminous fluxes travel. Thus, the luminous fluxes incident on the firstcylindrical lens 205 are compressed in the horizontal direction in FIG.1, and are basically guided to the light modulation panel 212 in anafocal state.

[0057] In the present embodiment, however, the beam compressor isintentionally provided with a predetermined amount of pupil distortionaberration which controls light intensity on the modulation surface ofthe light modulation panel 212 as an illumination surface to haveuniform or arbitrary distribution. The effects of the pupil distortionaberration of the beam compressor are later described with reference toFIG. 5.

[0058] The luminous fluxes transmitted through the first cylindricallens 205 are incident on the cylindrical condenser lens 209. Asdescribed above, the cylindrical condenser lens 209 superimposes theluminous fluxes in the vertical direction in FIG. 1 for integration onthe modulation surface of the light modulation panel 212 located at theposition of the focal line of the cylindrical condenser lens 209.

[0059] The luminous fluxes transmitted through the cylindrical condenserlens 209 are incident on the second cylindrical lens 210. The secondcylindrical lens 210 has a refractive power only in the horizontaldirection in FIG. 1 and forms the beam compressor as a pair with thefirst cylindrical lens 205 as described above. Thus, the luminous fluxesare compressed in the horizontal direction in FIG. 1 and guided to thelight modulation panel 212 in an afocal state.

[0060] The second cylindrical lens 210 is arranged to dispose a pupil ofthe first cylindrical lens 205 and the modulation surface of the lightmodulation panel 212 generally in an optically conjugate relationship(the optically conjugate relationship has low accuracy due to theaberration intentionally provided for the beam compressor). The pupil ofthe first cylindrical lens 205 is thus imaged on the modulation surfaceof the light modulation panel 212 in the horizontal direction in FIG. 1.

[0061] The second cylindrical lens 210 is arranged for correcting thedivergence of the luminous fluxes emitted from the lamp unit formed ofthe light source lamp 200 and the parabolic reflecting mirror 201 toreliably guide the luminous fluxes transmitted through the pupil of thefirst cylindrical lens 205 to the modulation surface of the lightmodulation panel 212, similarly to the function of the secondcylindrical lens array homogenizer 207.

[0062] The luminous fluxes transmitted through the cylindrical lens 210are incident on a dummy polarization beam splitter 211. The dummy may beformed of a dichroic prism or mirror instead. Whether the dummy isformed of a polarization beam splitter or a dichroic prism, thepolarization direction of the luminous fluxes is set to the verticaldirection in FIG. 1.

[0063] As described above, the luminous fluxes transmitted through theillumination optical system of the present embodiment are guided to thelight modulation panel 212. The illumination characteristics of thepresent embodiment are later described.

[0064] (About Light Source Lamp (DC Drive Type Discharge Gas ExcitingArc Tube) 200)

[0065] Next, description is made for a luminous flux emitted from thelamp unit and incident on the polarization conversion element 208 withreference to FIGS. 3(A) and 3(B).

[0066]FIG. 3(A) shows, in a simplified form, the lamp unit formed of thelight source lamp 200 and the parabolic reflecting mirror 201, the mask208 d provided for the polarization conversion element 208, and thefirst cylindrical lens array homogenizer 206 which is disposed betweenthe lamp unit and the mask 208 d. The ultraviolet ray cut filter 202 andthe second cylindrical lens array homogenizer 207 are omitted in FIG.3(A).

[0067] The light source lamp 200 is formed such that a discharge end ofa cathode electrode 200 a is disposed at the focal point of theparabolic reflecting mirror 201 and an absorption end of an anodeelectrode 200 b is disposed ahead away from the discharge end of thecathode electrode 200 a by a predetermined discharge gap.

[0068] When a DC bias is applied to the light source lamp 200, electronsemitted from the side of the cathode electrode 200 a excite the gas toproduce a light emission area 200 c with high luminance near thedischarge end of the cathode electrode 200 a. A light emission area 200d with low luminance is also formed between both electrodes 200 a and200 b.

[0069] Light emitted from the light emission area 200 c obliquelybackward is reflected by a portion of the parabolic reflecting mirror201 near the light source lamp 200 and directed toward the firstcylindrical lens array homogenizer 206 while it is diverged at adivergence angle θ_(a2). Then, the luminous flux emerging from the firstcylindrical lens array homogenizer 206 is incident on the polarizationconversion element 208 from the slit aperture 208 e between the mask 208d while it is converged.

[0070] On the other hand, light emitted from the light emission area 200c obliquely forward is reflected by a portion of the parabolicreflecting mirror 201 away from the light source lamp 200 and directedtoward the polarization conversion element 208 via the first cylindricallens array homogenizer 206 while it is diverged at a divergent angleθ_(a1).

[0071] The slit aperture 208 e in the mask 208 d of the polarizationconversion element 208 is disposed at the position away from the firstcylindrical lens array homogenizer 206 by the focal length f thereof.

[0072]FIG. 4(A) shows a luminous flux incident on the polarizationconversion element 208 when a light source lamp 250 of an AC drive typeis used in the illumination optical system shown in FIG. 3(A).

[0073] The light source lamp 250 is formed such that the focal point ofthe parabolic reflecting mirror 201 is positioned between a dischargeend of an electrode 250 a and a discharge end of an electrode 250 b.When an AD bias is applied to the light source lamp 250, a gas isexcited between both electrodes 250 a and 250 b to produce two lightemission areas 250 c 1 and 250 c 2 with high luminance near thedischarge end of the electrode 250 a and the discharge end of theelectrode 250 b. A light emission area 250 d with low luminance is alsoformed between both electrodes 250 a and 250 b.

[0074] Light emitted from the two light emission areas 250 c 1 and 250 c2 obliquely backward is reflected by a portion of the parabolicreflecting mirror 201 near the light source lamp 250 and directed towardthe first cylindrical lens array homogenizer 206 while it is diverged ata divergence angle θ_(b2). Then, the luminous flux emitted from thefirst cylindrical lens array homogenizer 206 is incident on thepolarization conversion element 208 from the slit aperture 208 e in themask 208 d while it is converged.

[0075] In FIG. 4(A), the slit aperture 208 e in the mask 208 d of thepolarization conversion element 208 is disposed at the position awayfrom the first cylindrical lens array homogenizer 206 by the focallength f thereof, similarly to FIG. 3(A).

[0076] On the other hand, light emitted from the two light emission area250 c 1 and 250 c 2 obliquely forward is reflected by a portion of theparabolic reflecting mirror 201 away from the light source lamp 250 anddirected toward the polarization conversion element 208 via the firstcylindrical lens array homogenizer 206 while it is diverged at adivergent angle θ_(b1).

[0077] In FIG. 3(A) and FIG. 4(A), the following relationships aresatisfied:

θ_(a1<θ) _(b1)

θ_(a2)<θ_(b2)

[0078]FIG. 4(B) shows the luminous flux toward the slit aperture 208 ein the mask 208 d when the light source lamp 250 which is the AD drivetype gas exciting lamp is used. Since the light source lamp 250 formsthe two light emission portions with high luminance, the luminous fluxhas intensity distribution which includes the peak intensity near theperiphery of the slit aperture 208 e. However, the divergent anglesθ_(b1) and θ_(b2) of the luminous flux are relatively large, so that alarge portion of the luminous flux toward the slit aperture 208 e isblocked by the mask 208 d. Thus, a large reduction in light amountoccurs due to the mask 208 d provided in the illumination opticalsystem.

[0079] On the other hand, FIG. 3(B) show the luminous flux toward theslit aperture 208 e in the mask 208 d in the present embodimentemploying the light source lamp 200 which is the discharge gas excitingarc tube of the DC drive type. Since the light source lamp 200 forms thesingle light emission portion with high luminance, so that the luminousflux has intensity distribution which includes the peak intensitygenerally at the center of the slit aperture 208 e and a lower intensitytoward an edge thereof. In other words, variations of light intensitydistribution of the luminous flux are reduced as compared with the caseshown in FIG. 4(B).

[0080] In addition, since the divergent angles θ_(a1) and θ_(a2) of theluminous flux are smaller than the divergent angles θ_(b1) and θ_(b2) inthe case shown in FIG. 4(A) when the light source lamp 250 of the ACdrive type is used, respectively, a smaller portion of the luminous fluxtoward the slit aperture 208 e is blocked by the mask 208 d.

[0081] The smaller portion of the luminous flux is essentially blockedby the mask 208 d as described above, and in addition, that blockedportion of the luminous flux has low light intensity. These facts makeit possible to significantly suppress a reduction in light amount due tothe mask 208 d as compared with the case where the discharge gasexciting arc tube of the AC drive type is used.

[0082] In this manner, the use of the discharge gas exciting arc tube ofthe DC drive type can suppress a reduction in light amount when theluminous flux is incident on the polarization conversion element 208 ascompared with the case where the discharge gas exciting arc tube of theAC drive type is used, thereby allowing improvement in light transferefficiency of the illumination optical system which employs thepolarization conversion element 208. In other words, the light from thelight source lamp 200 can be utilized with high efficiency.

[0083] (About Optical Integrator)

[0084] Next, description is made for an optical integrator used in theillumination optical system described above with reference to FIG. 5.

[0085] In optics disposed in FIG. 5, a first cylindrical lens arrayhomogenizer 306 corresponds to the first cylindrical lens arrayhomogenizer 206 in FIG. 1. A second cylindrical lens array homogenizer307 corresponds to the second cylindrical lens array homogenizer 207 inFIG. 1.

[0086] A cylindrical condenser lens 309 corresponds to the cylindricalcondenser lens 209 in FIG. 1.

[0087] The first and second cylindrical lens array homogenizers 306 and307, and the cylindrical condenser lens 309 constitute the opticalintegrator.

[0088] A light modulation panel 312 in FIG. 5 corresponds to the lightmodulation panel 212 in FIG. 1.

[0089] A luminous flux indicated by an outline arrow in FIG. 5, which isguided to the first cylindrical lens array homogenizer 306 and generallycollimated in an optical integration direction, is split by pupils oflenses in the array in the vertical direction (first axis direction) inFIG. 5 and condensed on respective focal lines. The positions of thefocal lines of the first cylindrical lens array homogenizer 306 areclose to the positions of pupils of the second cylindrical lens arrayhomogenizer 307, and are arranged such that the incident luminous fluxindicated by the outline arrow is hardly subjected to a refractiveeffect by the second cylindrical lens array homogenizer 307 when theluminous flux is completely collimated ideal light.

[0090] Each luminous flux transmitted through the second cylindricallens array homogenizer 307 is guided to the cylindrical condenser lens309. Since the optical axis of each luminous flux is shifted from theoptical axis of the cylindrical condenser lens 309, the optical axes ofthe respective luminous fluxes split by the pupils of the lenses of thefirst cylindrical lens array homogenizer 306 are condensed at theposition of a focal line of the cylindrical condenser lens 309.

[0091] The distance between the principal planes of the firstcylindrical lens array homogenizer 306 and the cylindrical condenserlens 309 is set to the sum of the focal length of the first cylindricallens array homogenizer 306 and the focal length of the cylindricalcondenser lens 309. Thus, each luminous flux split by the pupil of eachlens of the first cylindrical lens array homogenizer 306 is transmittedthrough the cylindrical condenser lens 309 to become collimated light incross section of FIG. 5.

[0092] The width of the collimated light is set to be enlarged at aratio of the focal length of the first cylindrical lens arrayhomogenizer 306 to the focal length of the cylindrical condenser lens309.

[0093] On the other hand, a modulation surface of the light modulationpanel 312 is disposed at the position of the focal line of thecylindrical condenser lens 309. This achieves an optical integrationoperation on the modulation surface of the light modulation panel 312.Consequently, the luminous flux incident on the illumination opticalsystem is converted to light having generally uniform light intensitydistribution which is irradiated to the modulation surface of the lightmodulation panel 312, irrespective of light intensity distribution atthe time of the incidence to the illumination optical system.

[0094] Next, description is made for the function of the secondcylindrical lens array homogenizer 307. The incident luminous fluxindicated by the outline arrow in FIG. 5 is not completely collimated.Especially, in the present embodiment which employs the light sourcelamp 200 with gas exciting light emission rather than a laser, the areafor excitation and light emission has a finite area on the order of 0.1mm at the minimum. Thus, even the use of a collimating lens or aparabolic reflecting mirror cannot provide a completely collimated beam,and the aforementioned incident luminous flux always includes divergence(a divergence angle).

[0095] The second cylindrical lens array homogenizer 307 is provided forcorrecting a blurred outline of an illumination area on the lightmodulation panel 312 due to the divergence error.

[0096] Description is hereinafter made with reference to FIG. 5. Theluminous fluxes split by the pupils of the lenses of the firstcylindrical lens array homogenizer 306 have divergence components fromthe entire pupil area. Thus, the pupil images of the lenses of the firstcylindrical lens array homogenizer 306 are projected and formed onto themodulation surface of the light modulation panel 312 by therecombination system formed of the second cylindrical lens arrayhomogenizer 307 and the cylindrical condenser lens 309.

[0097] The position of a focal line on the side of light incidence ofeach lens of the second cylindrical lens array homogenizer 307 is set tothe pupil position of each lens of the first cylindrical lens arrayhomogenizer 306. The divergence components of the luminous fluxes splitby the pupils of the lenses of the first cylindrical lens arrayhomogenizer 306 are shown by fine dotted lines in FIG. 5. The luminousfluxes split by the pupils of the lenses of the first cylindrical lensarray homogenizer 306 are transmitted through the second cylindricallens array homogenizer 307 and thus converted into collimated light incross section of FIG. 5. The collimated light is condensed to the focalline plane of the cylindrical condenser lens 309 by the cylindricalcondenser lens 309.

[0098] In other words, the pupil images of the lenses of the firstcylindrical lens array homogenizer 306 are superimposed and formed intoimages in an optically integrated state on the modulation surface of thelight modulation panel 312. Thus, the modulation surface of the lightmodulation panel 312 is illuminated by light which has intensitydistribution with sharp edges in cross section of FIG. 5.

[0099] (About Optics For Converting Light Intensity Distribution)

[0100] Next, description is made for the optics which convert lightintensity distribution incorporated in the illumination optical systemof the aforementioned embodiment with reference to FIG. 6.

[0101] In the optics in FIG. 6, a first cylindrical lens 405 correspondsto the first cylindrical lens 205 in FIG. 1, and a second cylindricallens 410 corresponds to the second cylindrical lens 210 in FIG. 1. Alight modulation panel 412 corresponds to the light modulation panel 212in FIG. 1.

[0102] A generally collimated luminous flux indicated by an outlinearrow in FIG. 6 is incident on the first cylindrical lens 405. The firstcylindrical lens 405 and the second cylindrical lens 410 disposed nextconstitute an afocal beam compressor of a convex-convex pair. Themagnification of beam compression is set such that the width of theincident luminous flux substantially matches the effective width of thelight modulation panel 412.

[0103] The spacing between the principal planes of the first cylindricallens 405 and the second cylindrical lens 410 is set to the sum of thefocal length of the first cylindrical lens 405 and the focal length ofthe second cylindrical lens 410. Thus, in cross section of the sheet ofFIG. 6, the luminous flux incident as the generally collimated lightemerges as generally collimated light with an angular magnificationcorresponding to the reciprocal of the compression magnification andirradiated to the light modulation panel 412.

[0104] On the other hand, the second cylindrical lens 410 has anotherfunction. The incident luminous flux indicated by the outline arrow inFIG. 6 is not completely collimated. Especially, in the presentembodiment which employs the light source lamp 200 with gas excitinglight emission source rather than a laser, the area for excitation andlight emission has a finite area on the order of 0.1 mm at the minimum.Thus, even the use of a collimating lens or a parabolic reflectingmirror cannot provide a completely collimated beam, and the incidentluminous flux always includes divergence (a divergence angle).

[0105] The second cylindrical lens 410 has the function of correcting ablurred outline of an illumination area on the light modulation panel412 due to the divergence error.

[0106] The first cylindrical lens 405 transmits the luminous flux withdivergence components from the entire pupil area thereof. The pupilimage of the first cylindrical lens 405 is projected and imaged onto amodulation surface of the light modulation panel 412 by the secondcylindrical lens 410.

[0107] The position of an image-forming conjugate line on the side oflight incidence of the second cylindrical lens 410 is set to the pupilposition of the first cylindrical lens 405. The position of animage-forming conjugate line on the side of light emergence of thesecond cylindrical lens 410 is set to the modulation surface of thelight modulation panel 412. The divergence component of the luminousflux from the pupil of the first cylindrical lens 405 is shown by finedotted lines in FIG. 6. Each luminous flux split by the pupil of thefirst cylindrical lens 405 is transmitted through the second cylindricallens 410 and thus condensed on the modulation surface of the lightmodulation panel 412 in cross section of FIG. 6. In other words, thepupil image of the second cylindrical lens 410 is transferred and formedinto an image on the modulation surface of the light modulation panel412.

[0108] The beam compressor formed of the first cylindrical lens 405 andthe second cylindrical lens 410 is an afocal optical system. Pupildistortion aberration, also referred to as spherical aberration in anafocal system, is intentionally provided as aberration caused by thepupil image transfer by the beam compressor. Each cylindrical surface ofthe first cylindrical lens 405 and the second cylindrical lens 410 has asmall curvature and is designed to produce more aberration with a shiftamount from the optical axis. As shown by coarse dotted lines in FIG. 6,rays close to the optical axis are transmitted through the secondcylindrical lens 410 and then converted to a slightly divergent luminousflux in cross section of the sheet of FIG. 6.

[0109] On the other hand, rays on the periphery of the pupil away fromthe optical axis are transmitted through the second cylindrical lens 410and then converted to a slightly convergent luminous flux in crosssection of the sheet of FIG. 6. Since the changes of divergence andconvergence are continuously and smoothly provided in this manner, theray density is low at the central portion and high at the peripheralportion on the modulation surface of the light modulation panel 412 incross section of the sheet of FIG. 6. Light intensity distributionilluminating the modulation surface of the light modulation panel 412 isprovided by multiplying light intensity distribution of the incidentluminous flux indicated with the outline arrow from the lamp unit formedof the gas exciting light source and the parabolic reflecting mirror bythe aforementioned ray density distribution.

[0110] Description is here made for light intensity distribution withwhich the light modulation panel is illuminated by using a combinationof the light intensity conversion optics and the optical integratordescribed in FIG. 5, with reference to FIGS. 7(A) to 7(C) and 8(A) and8(B).

[0111] FIGS. 7(A) to 7(C) and 8(A) and 8(B) show the process of forminglight intensity distribution on the light modulation panel by theillumination optical system of the present embodiment. FIGS. 7(A) and8(A) show a cross sectional profile of the luminous flux emitted fromthe lamp unit formed of the light source lamp 200 and the parabolicreflecting mirror 201. In FIG. 7(A), a brighter portion indicates ahigher light intensity. In FIG. 8(A), a solid line shows light intensitydistribution in cross section in a horizontal (X) direction at thecenter (0 mm) in a vertical (Y) direction (that is, a short sidedirection of the rectangular light modulation panel corresponding to afirst axis direction) in FIG. 7(A), while a dotted line shows lightintensity distribution in cross section in the vertical (Y) direction atthe center (0 mm) in the horizontal (X) direction (that is, a long sidedirection of the rectangular light modulation panel corresponding to asecond axis direction) in FIG. 7(A).

[0112] The light intensity distribution of the luminous flux shown inFIGS. 7(A) and 8(A) is divided and integrated by the optical integratorin areas sectioned by horizontal lines in FIG. 7(A). Then, the lightintensity distribution is multiplied in the direction of the lightintensity conversion optics by ray density distribution on themodulation surface of the light modulation panel (light modulation panelsurface) shown in FIG. 7(B) resulting from the aforementioned pupildistortion aberration of the beam compressor to provide light intensitydistribution on the modulation surface of the light modulation panelshown in FIGS. 7(C) and 8(B).

[0113] As can be seen from FIGS. 7(C) and 8(B), the light intensitydistribution of the illumination luminous flux incident on themodulation surface of the light modulation panel has high intensity andis generally flat (uniform).

[0114] It goes without saying, however, that the ray densitydistribution on the modulation surface of the light modulation panel canbe changed in accordance with a purpose by designing the pupildistortion aberration of the beam compressor to a predetermined value.In this manner, the illumination luminous flux incident on themodulation surface of the light modulation panel can be intentionallyprovided with predetermined light intensity distribution in thedirection in which the light intensity conversion optics exert theeffect.

[0115] Next, characteristics provided by the illumination optical systemexplained so far are described with reference to FIGS. 9(A) to 9(C) and10(A) to 10(C).

[0116] FIGS. 10(A) to 10(C) show incident angle distribution of rayssubjected to an optical integration operation by a conventional pair oftwo-dimensional fly eye lens arrays on an illumination surface such as amodulation surface of a light modulation panel. In FIG. 10(A), the outerperiphery of a circle corresponds to azimuth angles of 360 degrees, andradial axes show elevation angles (angles of incidence) with respect tothe normal to the illumination surface (a perpendicular incident axis).In FIG. 10(A), the outer periphery is divided by the radial axes inelevation angles of 20 degrees. FIGS. 10(B) and 10(C) show lightintensity distribution taken along a line B-B and a line C-C in FIG.10(A), respectively.

[0117] On the other hand, FIGS. 9(A) to 9(C) show incident angledistribution of rays provided by the illumination optical system of thepresent embodiment on an illumination surface such as a modulationsurface of a light modulation panel. In FIG. 9(A), the outer peripheryof a circle corresponds to azimuth angles of 360 degrees, and radialaxes show angles of incidence on the illumination surface. In FIG. 9(A),the outer periphery is divided by the radial axes in elevation angles of20 degrees. FIGS. 9(B) and 9(C) show light intensity distribution in aB-B section direction (B-B axis direction corresponding to a first axisdirection) and a C-C section direction (C-C axis direction correspondingto a second axis direction) in FIG. 9(A), respectively.

[0118] As can be seen from comparison between FIG. 9(A) and 10(A), theluminous flux irradiated to the illumination surface can have generallyuniform light intensity distribution on the illumination surface in bothcases. However, a large difference is found between them in incidentangle characteristics of a luminous flux.

[0119] Specifically, as shown in FIG. 10(A), the illumination luminousflux subjected to the optical integration operation by the pair oftwo-dimensional fly eye lens arrays has symmetrical ray distribution intwo directions of the azimuth on the illumination surface.

[0120] In contrast, in the present embodiment, as shown in FIG. 9(A),elevation angles in the optical integration direction (B-B sectiondirection) vertical in FIG. 9(A) are similar to those in FIG. 10(A),while in the direction in which the light intensity conversion opticsexert the effect (C-C section direction), no luminous fluxes aresuperimposed by the optical integration operation, so that elevationangles dependent on the angular magnification determined by thecompression magnification of the beam compressor are provided withrespect to the divergent angle of the luminous flux emitted from thelamp unit. Thus, the ray incident angle on the illumination surface canbe significantly reduced in the direction in which the light intensityconversion optics exert the effect.

[0121] Specifically, in the intensity distribution of illumination lighton the illumination surface varying depending on the deviation angle ofthe incident ray with respect to the normal to the illumination surface,a ratio α:β is an aspect ratio of 2:1 or higher, where α and β representangle widths at which light intensity reaches half of a peak value P(½P) in each of two (B-B axis and C-C axis) directions orthogonal toeach other on the illumination surface.

[0122] More specifically, the angle width at which light intensityreaches half of the peak value on the B-B axis is twice or more theangle width at which the light intensity reaches half of the peak valueon the C-C axis. Alternatively, the maximum value of the angle width atwhich the light intensity reaches half of the peak value in the B-B axisdirection may be set to be twice or more the maximum value of the anglewidth at which the light intensity reaches half of the peak value in theC-C axis direction.

[0123] Description is hereinafter made for influences (advantages)exerted by the aforementioned characteristics on a projection displayapparatus which employs the illumination optical system described abovein Embodiment 2.

[0124] (Embodiment 2)

[0125]FIG. 11 shows the overall optical system in a projection displayapparatus which is Embodiment 2 of the present invention.

[0126] In FIG. 11, reference numeral 1 schematically shows theillumination optical system described in Embodiment 1. A representationon the left in the frame in the figure shows the illumination opticalsystem on the right viewed from an arrow D.

[0127] Reference numerals 2R, 2G, and 2B show reflection type liquidcrystal modulation panels (hereinafter referred to as liquid crystalmodulation panels) for read, green, and blue, respectively. Referencenumeral 3 shows a light modulation panel driver which converts anexternal video input signal from an image information supply apparatussuch as a personal computer, a television, a VCR, and a DVD player, notshown, into a driving signal for driving the liquid crystal modulationpanels 2R, 2G, and 2B. Each of the liquid crystal modulation panels 2R,2G, and 2B forms an original image with liquid crystal corresponding tothe driving signal input thereto to reflect and modulate an illuminationluminous flux incident on each of the liquid crystal modulation panels2R, 2G, and 2B.

[0128] Of illumination light as linearly polarized light polarized in adirection orthogonal to the sheet of FIG. 11 from the illuminationoptical system 1, a light component of magenta (red and blue) is firstreflected by a magenta splitting dichroic mirror 30 which reflects thelight component of magenta and transmits a light component of green.

[0129] The reflected light component of magenta is incident on a bluecross color polarizer 34 which provides a phase difference of π forpolarized light of blue. This produces a light component of blue whichis linearly polarized light polarized in a direction in parallel withthe sheet and a light component of red which is linearly polarized lightpolarized in the direction orthogonal to the sheet.

[0130] The blue light component and the red light component are incidenton a polarization beam splitter 33 in which the blue light componentthat is P-polarized light is transmitted through a polarizationsplitting film of the polarization beam splitter 33 and guided to theliquid crystal modulation panel 2B for blue. The red light componentwhich is S-polarized light is reflected by the polarization splittingfilm of the polarization beam splitter 33 and guided to the liquidcrystal modulation panel 2R for red.

[0131] On the other hand, the green light component transmitted throughthe magenta splitting dichroic mirror 30 is transmitted through a dummyglass 36 for correcting an optical path length and incident on apolarization beam splitter 31.

[0132] The green light component which is S-polarized light incident onthe polarization beam splitter 31 is reflected by a polarizationsplitting film of the polarization beam splitter 31 and guided to theliquid crystal modulation panel 2G for green.

[0133] In this manner, the respective liquid crystal modulation panels2R, 2G, and 2B are illuminated by the corresponding color lightcomponents.

[0134] The illumination light components for the respective colorsincident on the liquid crystal modulation panels 2R, 2G, and 2B (thelinearly polarized light polarized in the direction orthogonal to thesheet) are given phase differences of polarization in accordance withthe modulation state of a pixel group arranged in the liquid crystalmodulation panels 2R, 2G, and 2B.

[0135] Of the modulated light emerging from the liquid crystalmodulation panels 2R, 2G, and 2B, light components polarized in the samedirection as the illumination light return toward the lamp unit alongthe optical path reversely to the illumination. Light componentspolarized in a direction orthogonal to the polarization direction of theillumination light reach a projection lens 4 as follows.

[0136] Specifically, the light modulated by the liquid crystalmodulation panel 2R for red is converted into P-polarized lightpolarized in the direction in parallel with the sheet and is transmittedthrough the polarization splitting film of the polarization beamsplitter 33. Next, the light is transmitted through a red cross colorpolarizer 35 which provides a phase difference of π for the polarizedlight for red, and is converted into a red light component as linearlypolarized light polarized in the direction orthogonal to the sheet.

[0137] The red light component which has been converted into S-polarizedlight is incident on a polarization beam splitter 32, reflected by apolarization splitting film thereof, and directed toward the projectionlens 4.

[0138] The light modulated by the liquid crystal modulation panel 2B forblue is converted into S-polarized light polarized in the directionorthogonal to the sheet and reflected by the polarization splitting filmof the polarization beam splitter 33. Then, the light is transmittedthrough the red cross color polarizer 35 without being subjected to theeffect of the polarizer 35 and incident on the polarization beamsplitter 32.

[0139] The blue light component which is S-polarized light is reflectedby the polarization splitting film of the polarization beam splitter 32and directed toward the projection lens 4.

[0140] The light modulated by the liquid crystal modulation panel 2G forgreen is converted into P-polarized light polarized in the direction inparallel with the sheet and transmitted through the polarizationsplitting film of the polarization beam splitter 31. The light istransmitted through a dummy glass 37 for correcting an optical pathlength and incident on the polarization beam splitter 32. The greenlight component which is P-polarized light is transmitted through thepolarization splitting film of the polarization beam splitter 32 anddirected toward the projection lens 4.

[0141] The liquid crystal modulation panels 2R, 2G, and 2B are adjustedor mechanically or electrically compensated for such that predeterminedpixels on the respective panels are relatively superimposed on a lightdiffusion screen 5 with high accuracy.

[0142] The three light components for the respective colors combined bythe polarization beam splitter 32 are taken by the entrance pupil of theprojection lens 4. The projection lens 4 is arranged to dispose amodulation surface of each liquid crystal modulation panel and adiffusion surface of the light diffusion screen 5 in an opticallyconjugate relationship. Thus, the light components for the respectivecolors combined by the polarization beam splitter 32 are transferred tothe light diffusion screen 5 to project and display a full-color imagecorresponding to the video signal on the light diffusion screen 5.

[0143] The dichroic film of the dichroic mirror 30 used in the opticalpath for illuminating the liquid crystal modulation panels 2R, 2G, and2B has the characteristic that, as a deviation amount of the incidentangle of a ray from 45 degrees with respect to the dichroic film isincreased, the splitting wavelength is shifted toward a shorterwavelength at an obtuse angle or toward a longer wavelength at an acuteangle.

[0144] Thus, when the optical integrator implemented by the conventionalpair of two-dimensional fly eye lens arrays is used in the illuminationoptical system, a luminous flux with the incident angle distribution asshown in FIG. 10(A) is incident on the dichroic film, so that luminousfluxes at different wavelengths coexist as the incident angle isdeviated from 45 degrees with respect to the dichroic film.

[0145] If the lamp unit serving as the light source has gradualradiation energy wavelength distribution like blackbody radiation, thedistribution of angles incident on the dichroic film is symmetric about45 degrees, so that the average cut wavelength is not changed. However,when the lamp unit of electron excited radiation which uses gas excitinglight emission is used as in the present embodiment, it has wavelengthspectral distribution including emission lines as dominant parts inradiation energy wavelength distribution, and thus the average cutwavelength is changed with the median point. Therefore, color splittingby the dichroic films is not appropriately achieved to result in thedisadvantage of poor color reproducibility in a projected image.

[0146] The polarization beam splitters 31, 32, and 33 used in theoptical path for illuminating the liquid crystal modulation panels 2R,2G, and 2B and the optical path for color combination are typicalpolarization beam splitters of a MacNeil type and have the polarizationsplitting films for S-polarized light and P-polarized light by means ofthe Brewster angle. As a deviation amount of the incident angle of a rayfrom 45 degrees with respect to the polarization splitting surface isincreased, accuracy of splitting of S-polarized light and P-polarizedlight is suddenly reduced.

[0147] The accuracy of splitting of S-polarized light and P-polarizedlight can be actually maintained at a ratio of approximately 50:1 when adeviation amount from 45 degrees falls within approximately ±3 degrees.Thus, when the optical integrator implemented by the conventional pairof two-dimensional fly eye lens arrays is used, a luminous flux with theincident angle distribution as shown in FIG. 10(A) is incident on thepolarization beam splitters 31 and 32. In addition, the luminous flux isincident on the polarization beam splitters 31, 32, and 33 after thereflection and polarization modulation by the liquid crystal modulationpanels 2R, 2G, and 2B. This produces the disadvantage that, of luminousfluxes with a deviation amount of incident angle of ±3 degrees or morefrom 45 degrees with respect to the polarization splitting surface, aportion of P-polarized light is reflected, and a portion of S-polarizedlight is transmitted.

[0148] The illumination light incident on each of the liquid crystalmodulation panels 2R, 2G, and 2B formed of reflection type liquidcrystal display element is given a phase difference of polarization inaccordance with the modulation state of pixels arranged in each of theliquid crystal modulation panels 2R, 2G, and 2B. However, even when theliquid crystal modulation panels 2R, 2G, and 2B send light which is notsubjected to a change in phase difference to display black, a luminousflux inclined three degrees or more from 45 degrees with respect to thepolarization splitting surface of the polarization beam splitters 31 to33 includes a portion of S-polarized light which is transmitted and aportion of P-polarized light which is reflected and transferred to thelight diffusion screen 5 through the projection lens 4. As a result, theintended black is displayed in gray to reduce illuminance contrast.

[0149] For the polarized light modulation characteristics of the liquidcrystal modulation panels 2R, 2G, and 2B, when twisted nematic liquidcrystal is used in the liquid crystal modulation panels, the liquidcrystal modulation panels 2R, 2G, and 2B fundamentally have thecharacteristic that it cannot accurately modulate light incident on thereflection type liquid crystal modulation panels at an azimuth of 45degrees. For this reason, in the optical integrator implemented by theconventional pair of two-dimensional fly eye lens arrays whichilluminates light from azimuths generally axial symmetric, polarizedlight modulation of the liquid crystal is not sufficient, and intendedblack display is shown in gray to reduce illuminance contrast, similarlyto the incident angle dependency characteristics of the polarizationbeam splitters 31 to 33 described above.

[0150] To address these disadvantages, the illumination optical system 1of Embodiment 1 described above can be used to provide a luminous fluxwith the incident angle distribution as shown in FIG. 9(A). When theluminous flux is incident on the dichroic mirror 30, a deviation amountof the incident angle from 45 degrees with respect to the dichroic filmfalls within +3 to 4 degrees. This can almost eliminate inappropriatecolor combination due to the change in the average cut wavelength withthe median point in the color splitting by the dichroic film to resultin poor color reproducibility in a projected image occurring when theoptical integrator implemented by the conventional pair oftwo-dimensional fly eye lens arrays is used.

[0151] When the luminous flux is incident on the polarization beamsplitter 31 to 33, a deviation amount of the incident angle from 45degrees with respect to each polarization splitting surface falls within±3 to 4 degrees. This can almost eliminate a reduction in illuminancecontrast due to the polarization splitting error which means thatpolarization splitting does not match the modulation state of the pixelsin the liquid crystal modulation panel occurring when the opticalintegrator implemented by the conventional pair of two-dimensional flyeye lens arrays is used.

[0152] For the disadvantage in providing the phase difference for themodulation of polarized light depending on the incident angle on thereflection type liquid crystal modulation panel, since almost nocomponents of illumination luminous fluxes are directed from an azimuthat which polarization modulation by the liquid crystal is notsufficiently achieved, a reduction in illuminance contrast can be almosteliminated.

[0153] The projection display apparatus of the present embodiment alsoprovides an advantage in the projection lens 4. Specifically, when thepolarization splitting direction or the wavelength splitting directionis set to the long side direction of the liquid crystal modulationpanel, it is possible to set the horizontal direction with narrowillumination angle distribution shown in FIG. 9(A) to the direction inwhich the projection lens 4 has a large projection field angle. This canreduce the width of a luminous flux transmitted in a direction in whichan aperture eclipse of the projection lens 4 called vignetting occurs.In other words, the advantage produces the effect of reducing vignettingdue to the pupil aperture eclipse of the projection lens 4 to prevent areduction in light amount at the edge of an image area projected on thelight diffusion screen 5, thereby producing a projected image of uniformlight intensity distribution.

[0154] It should be noted that, while the present embodiment descriedabove employs the reflection type screen to form the image displaysystem, the screen may be of the reflection type or a transmission type.Specifically, when a screen with predetermined diffusion is used, aprojection display apparatus can function to allow a user to directlyview the screen 5 to recognize a projected image.

[0155] The structure of the projection display optical system describedin Embodiment 2 described above is only illustrative, and theillumination optical system of the present invention is applicable to aprojection display optical system other than Embodiment 2.

[0156] Each of Embodiments 1 and 2 has been described for the examplewhich employs, as the optical integrator, the first cylindrical lensarray homogenizer 206 and the second cylindrical lens array homogenizer207 which perform splitting and recombination of the illuminationluminous flux incident as the generally collimated luminous flux fromthe light source by using the lens array in the first axis direction(the vertical direction) on the section generally orthogonal to thetraveling direction of the luminous flux, and as the light source, thedischarge gas exciting arc tube (the light source lamp) 200 of the DCdrive type. However, the present invention is applicable to a structurewhich employs, as the optical integrator, a two-dimensional lens arraywhich performs splitting and recombination of an illumination luminousflux incident as a generally collimated illumination luminous flux fromthe light source in the first and second axis directions (the verticaland horizontal directions) on a section generally orthogonal to thetraveling direction of the luminous flux, and as the light source, thedischarge gas exciting arc tube (the light source lamp) 200 of the DCdrive type.

[0157] As described above, according to Embodiments 1 and 2, in theillumination optical system which provides an illumination luminous fluxwith a small incident angle in one axis direction on a section of theillumination luminous flux, the discharge gas exciting arc tube of theDC drive type is used as the light source. This can maintain thecharacteristic of the illumination optical system of improving thepolarization splitting (and recombination) properties in thepolarization splitting (and recombination) element and the spatial lightmodulator on which the illumination luminous flux is incident, while theintensity distribution of the luminous flux toward the polarizationconversion element can be prevented from enlarging, thereby making itpossible to reduce a light amount blocked by the mask (that is, areduction in light amount) provided for the polarization conversionelement. It is thus possible to enhance light transfer efficiency of theillumination optical system which uses the polarization conversionelement.

[0158] Such an illumination optical system can be used in a projectiondisplay optical system or a projection display apparatus to improvebrightness of a projected image without increasing an amount of lightemitted by the light source and to achieve a high contrast.

[0159] The illumination optical system of each of Embodiments 1 and 2illuminates an illumination surface with a generally telecentricillumination luminous flux. In intensity distribution of illuminationlight on the illumination surface changing depending on a deviationangle of an incident ray with respect to a normal to the illuminationsurface, a ratio of angle widths at which light intensity reaches halfof a peak value in each of two axis directions orthogonal to each otheron the illumination surface is an aspect ratio of 2:1 or higher.Alternatively, in the illumination optical system, in intensitydistribution of illumination light on the illumination surface changingdepending on a deviation angle of an incident ray with respect to anormal to the illumination surface, the maximum value of the angle widthat which light intensity reaches half of the peak value in one of thetwo axis directions orthogonal to each other on the illumination surfaceis twice or more the maximum value of the angle width at which lightintensity reaches half of the peak value in the other direction.

[0160] With these structures and settings, it is possible to realize anillumination optical system which can use light from a light source withhigh efficiency and provide an illumination luminous flux with highuniform illuminance in addition to the aforementioned effects. Theillumination optical system can be used in a projection display opticalsystem to provide a projected image with high brightness and a highcontrast.

[0161] While preferred embodiments have been described, it is to beunderstood that modification and variation of the present invention maybe made without departing from the scope of the following claims.

What is claimed is:
 1. An illumination optical system comprising: alight source; an optical integrator which uses a lens array to performsplitting of a luminous flux incident as a generally collimated luminousflux from the light source in a first axis direction in atwo-dimensional section orthogonal to a traveling direction of theluminous flux; and a polarization conversion element which includes apolarization beam splitter array, a plurality of ½ wave plates, and amask, the polarization beam splitter array having a plurality ofpolarization beam splitters arranged in multiple stages corresponding toa plurality of predetermined lens areas in the lens array, each of the ½wave plates rotating a polarization direction of first polarized lightsubstantially 90 degrees out of the first and second polarized lightwith polarization directions orthogonal to each other split by each ofthe polarization beam splitters, and the mask covering a plurality ofareas out of incident surfaces of the polarization beam splitter arrayto prevent incident of the second polarized light on each of the ½ waveplates, wherein the light source is a discharge gas exciting arc tube ofa DC drive type.
 2. The illumination optical system according to claim1, wherein the mask has light-transmitting portions, and a luminous fluxtransmitted through each light-transmitting portion of the mask haslight intensity distribution including a higher light intensity in acentral portion than a peripheral portion thereof.
 3. The illuminationoptical system according to claim 1, wherein the illumination opticalsystem illuminates an illumination surface in a generally rectangularshape, and the first axis direction is a short side direction of theillumination surface.
 4. The illumination optical system according toclaim 1, further comprising optical intensity converting member forconverting light intensity distribution in a second axis directionorthogonal to the first axis direction on the two-dimensional section.5. The illumination optical system according to claim 1, wherein theillumination optical system illuminates an illumination surface with agenerally telecentric luminous flux, and light intensity of the luminousflux on the illumination surface varies depending on a deviation angleof an incident ray with respect to a normal to the illumination surface,and the illumination optical system illuminates the illumination surfacesuch that, in the light intensity distribution, a ratio of angle widthsat which light intensity reaches half of a peak value in each of twoaxis directions orthogonal to each other on the illumination surface isan aspect ratio of 2:1 or higher.
 6. The illumination optical systemaccording to claim 5, wherein, in the light intensity distribution, aratio of an angle width at which light intensity reaches half of a peakvalue in a second axis direction orthogonal to the first axis directionto an angle width at which light intensity reaches half of a peak valuein the first axis direction is an aspect ratio of 2:1 or higher.
 7. Theillumination optical system according to claim 1, wherein theillumination optical system illuminates an illumination surface with agenerally telecentric luminous flux, and light intensity of the luminousflux on the illumination surface varies depending on a deviation angleof an incident ray with respect to a normal to the illumination surface,and in the light intensity distribution, a maximum value of an anglewidth at which light intensity reaches half of a peak value in one oftwo axis directions orthogonal to each other on the illumination surfaceis twice or more a maximum value of an angle width at which lightintensity reaches half of a peak value in the other direction.
 8. Theillumination optical system according to claim 7, wherein, in the lightintensity distribution, a maximum value of an angle width at which lightintensity reaches half of a peak value in a second axis directionorthogonal to the first axis direction is twice or more a maximum valueof an angle width at which light intensity reaches half of a peak valuein the first axis direction.
 9. A projection display optical systemcomprising: the illumination optical system according to claim 1; aspatial light modulator which modulates a luminous flux emerging fromthe illumination optical system by a group of pixels arrangedtwo-dimensionally; and a projection lens which projects the luminousflux modulated by the spatial light modulator onto a projection surface.10. A projection display apparatus comprising: a light source which is adischarge gas exciting arc tube of a DC drive type; an opticalintegrator which uses a lens array to perform splitting of a luminousflux incident as a generally collimated luminous flux from the lightsource in a first axis direction in a two-dimensional section orthogonalto a traveling direction of the luminous flux; a polarization conversionelement which includes a polarization beam splitter array, a pluralityof ½ wave plates, and a mask, the polarization beam splitter arrayhaving a plurality of polarization beam splitters arranged in multiplestages corresponding to a plurality of predetermined lens areas in thelens array, each of the ½ wave plates rotating a polarization directionof first polarized light substantially 90 degrees out of the first andsecond polarized light with polarization directions orthogonal to eachother split by each of the polarization beam splitters, and the maskcovering a plurality of areas out of an incident surface of thepolarization beam splitter array to prevent incident of the secondpolarized light on each of the ½ wave plates; a spatial light modulatorwhich modulates a luminous flux emerging from the illumination opticalsystem by a group of pixels arranged two-dimensionally; and a projectionlens which projects the luminous flux modulated by the spatial lightmodulator onto a projection surface.
 11. An image display systemcomprising: the projection display apparatus according to claim 10; anda screen which forms the projection surface, wherein the image displaysystem allows an observer to observe a projected image with one ofdivergent reflection light from the screen and divergent transmissionlight through the screen, each light having predetermined directivity.12. An illumination optical system comprising: a light source in which acathode electrode and an anode electrode are provided, and by applying aDC voltage a discharge gas is excited and light is emitted from thevicinity of the cathode electrode; a lens array in which a plurality oflenses are arranged in a first direction substantially orthogonal to anillumination direction, each lens condensing a part of a luminous fluxfrom the light source in the first direction; and a mask in whichlight-transmitting portions transmitting luminous fluxes condensed bythe lenses and light-blocking portions blocking the luminous fluxescondensed by the lenses are arranged alternately in the first direction.13. A projection display apparatus comprising: a light source in which acathode electrode and an anode electrode are provided, and by applying aDC voltage a discharge gas is excited and light is emitted from thevicinity of the cathode electrode; a lens array in which a plurality oflenses are arranged in a first direction substantially orthogonal to anillumination direction, each lens condensing a part of a luminous fluxfrom the light source in the first direction; a mask in whichlight-transmitting portions transmitting luminous fluxes condensed bythe lenses and light-blocking portions blocking the luminous fluxescondensed by the lenses are arranged alternately in the first direction;a polarization beam splitter array in which first polarization beamsplitters and second polarization beam splitters are arrangedalternately in the first direction, each first polarization beamsplitter reflecting a first polarized light out of transmitted lightthrough the light-transmitting portion and transmitting a secondpolarized light out of the transmitted light, the polarization directionof the second polarized light being rotated by substantially 90 degreesfrom the polarization direction of the first polarized light, eachsecond polarization beam splitter reflecting the first polarized lightreflected by the first polarization beam splitter in a directionsubstantially parallel to the transmitting direction of the secondpolarized light; wave plates which rotate the polarization direction ofthe first polarized light from the second polarization beam splitterssubstantially 90 degrees; a light modulator which modulates the secondpolarized light at a substantial rectangular area having a short side inthe first direction; and a projection optical system which projectsmodulated light by the light modulator.
 14. An image display systemcomprising: the projection display apparatus according to claim 13; anda screen which has a projection surface.